The invention relates to doctor blades for metering electrophotographic toner held on a developer roller by physically contacting a sector of the roller with a surface of the blade. More particularly, the invention is directed to a flexible doctor blade having a radiused surface for contacting the developer roll surface instead of a conventional flat surface. A radiused nub extends from the blade body which allows the pressure needed in the doctoring nip to be achieved with a lower force between the blade and the developer roll.
In electrophotographic printers that use a developer roll to carry toner to the photoconductor, it is necessary to provide a means of metering the toner to produce a thin, uniform layer of toner on the developer. FIG. 1 shows a typical developer. Toner 1 is coated on the developer roll 2 by a toner adder roll 3. As the developer roll rotates it carries the toner to a doctor blade 4 which is pressed against the developer roll by a doctor blade spring 5. The pressure that is generated in the nip between the doctor blade and developer causes the formation of a layer of toner 6 that is then carried to the photoconductor 7 where the latent image is developed. The amount and uniformity, of toner that passes under the doctor blade is a function of, among other things, the pressure in the nip with higher pressures producing less mass per unit area. The torque needed to turn the developer roll is a function of the force of the doctor blade spring and the coefficient of friction between the doctor blade and developer roll.
The doctor blades used in the IBM/Lexmark 40.times.9 printer family all use a flat steel doctor blade and rubber or urethane developer rolls. These components are controlled to tight straightness and runout tolerances in order to control the uniformity of toner flow. The doctor blade force used is up to 1400 grams with the resulting frictional torque being more than half of the developer's required torque. The 40.times.7 laser printer uses a flat sandpaper blade with a foam backing and a load of 800 grams. (See U.S. Pat. Nos. 5,708,943; issued Jan. 13, 1998; 5,702,812; issued Dec. 30, 1997; and 5,623,718; issued Apr. 22, 1997, all assigned to the assignee of this invention.) The compliance of the foam backing reduces tolerances and costs of the components, but the frictional torque is still the primary source of torque in the developer. In order to extend the technology to higher speeds, it will be beneficial to reduce the developer heating that is caused by doctor blade friction.
Both the flat steel blade and flat sandpaper flex blade have shown problems when used with 8 micron toners. Toner becomes packed in the forward region of flat doctor blade shown in FIGS. 2A & 2B. The wedge of toner becomes compacted to the point that its cohesive strength and adhesion to the blade can produce a blockage of toner. This prevents toner from entering the nip and results in vertical white streaks in print.
One solution to the difficulties encountered was the use of a compliant doctor blade having a conductive metal bar with a lower surface to which a resilient layer is attached. The lower surface of the resilient layer has attached to it a stiff shim. The stiff shim has a bottom layer of conductive and abrasive material. At one end of the doctor blade a resilient conductive coating bridges the abrasive layer and the metal bar. If desired, the blade is at an angle with the surface of the developer roller--see U.S. Pat. No. 5,797,076, issued Aug. 18, 1998, assigned to the assignee of this invention.
A second solution to the problem was disclosed in U.S. Pat. Nos. 5,400,128 and 5,237,375, which were directed to wiper and spreader blades with conductive coatings. The wiper blade removed excess toner from the photosensitive drum and the spreader blade controls and adjusts the quantity of toner on the developing cylinder. Nowhere is there disclosed or suggested the novel doctor blade of the present invention.
It is an object of the present invention to provide a doctor blade for use in an electrophotographic process which consists of a radiused surface which lowers torque exerted by the developer roller.
It is a further object of the present invention to provide a doctor blade for use in an electrophotographic process which has a relatively small radius surface of a flexible conductive material to meter toner onto a developer roller.
Still, another objective of the present invention is to provide a doctor blade for use in an electrophotographic process which is less sensitive to developer roll and doctor blade tolerances.
Yet another objective of the present invention is to provide a doctor blade for use in an electrophotographic process which is less prone to toner streaking.